In a construction machine such as a hydraulic excavator having an upperstructure provided with an exterior cover, working equipment attached to the upperstructure and including a boom, arm, bucket and the like, and hydraulic actuators including a swing motor for driving the upperstructure and an actuator for driving the working equipment, a steel-made hydraulic oil tank, specifically a steel-made hydraulic oil tank adapted to store pressure oil to be fed to the above-mentioned hydraulic actuators is mounted in many instances from the viewpoint of the assurance of strength. When a hydraulic oil tank is mounted together with an engine, hydraulic pumps, control valves, a swing motor and the like in an extremely narrow, equipment installation space on an upperstructure as in a compact excavator, a hydraulic oil tank made of synthetic resin may, however, be mounted for its easy moldability into a complex shape. Compared with a steel-made hydraulic oil tank, such a hydraulic oil tank made of synthetic resin has advantages that it requires neither painting nor anti-rust treatment, is light weight, and allows less transmission of heat from the hydraulic oil stored inside to the outside.
Such a hydraulic oil tank made of synthetic resin is mounted such that its side wall opposes the exterior cover of the upperstructure, generally with a predetermined clearance maintained relative to the exterior cover to avoid interferences or contact with the exterior cover by bumping or the like of the body. Further, this hydraulic oil tank is provided with a gage comprised of a transparent member that enables a visual check on the level of hydraulic oil stored inside through a hole portion formed in the exterior cover. The visual check on the level of hydraulic oil via such a gage is to ascertain whether or not the amount of the hydraulic oil stored in the hydraulic oil tank has gone down as a result of an operation over a long time. When the hydraulic oil in the hydraulic oil tank has gone down beyond an acceptable value, the replacement or the like of the hydraulic oil in the hydraulic oil tank is performed, for example. A hydraulic oil tank made of synthetic resin, which is of this type, is disclosed, for example, in Patent Document 1.